/*
 * (c)2012 Michael Duane Rice All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 * Redistributions of source code must retain the above copyright notice, this
 * list of conditions and the following disclaimer. Redistributions in binary
 * form must reproduce the above copyright notice, this list of conditions
 * and the following disclaimer in the documentation and/or other materials
 * provided with the distribution. Neither the name of the copyright holders
 * nor the names of contributors may be used to endorse or promote products
 * derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

/* $Id: iso_week_date_r.c 2394 2013-05-04 10:26:24Z swfltek $ */

/*
    Compute the ISO 8601 week date corresponding to the given year and day of year.
    See http://en.wikipedia.org/wiki/ISO_week_date for a full description. To summarize:

        Weeks are numbered from 1 to 53.
        Week days are numbered 1 to 7, beginning with Monday as day 1.

        The first week of the year contains the first Thursday in that year.
        Dates prior to week 1 belong to the final week of the previous year.

        The final week of the year contains the last Thursday in that year.
        Dates after the final week belong to week 1 of the following year.

*/

#include "time.h"

void
iso_week_date_r(int y, int yday, struct week_date * iso)
{
	uint16_t        years, n, wday;
	int             weeknum;
	int             isLeap;

	iso->year = y;

	isLeap = is_leap_year(y);

	/* compute days elapsed since epoch */
	years = y - 2000;
	n = 365 * years + yday;
	if (years) {
		n++;		/* epoch was a leap year */
		n += years / 4;
		n -= isLeap;
		if (years > 100)
			n--;
	}
	/* compute ISO8601 day of week (1 ... 7, Monday = 1) */
	wday = n + 6;		/* epoch was a Saturday */
	wday %= 7;
	if (wday == 0)
		wday = 7;

	iso->day = wday;

	/* compute tentative week number */
	weeknum = yday + 11 - wday;
	weeknum /= 7;

	/* if 53, it could be week 1 of the following year */
	if (weeknum == 53) {
		/*
	            The final week must include its Thursday in the year. We determine the yday of this
	            weeks Thursday, and test whether it exceeds this years length.
	        */

		/* determine final yday of this year, 364 or 365 */
		n = 364 + isLeap;

		/* compute yday of this weeks Thursday */
		wday--;		/* convert to zero based week, Monday = 0 */
		yday -= wday;	/* yday of this weeks Monday */
		yday += 3;	/* yday of this weeks Thursday */

		/* Is this weeks Thursday included in the year? */
		if (yday > (int) n) {
			iso->year++;
			weeknum = 1;
		}
	}
	iso->week = weeknum;

	/*
            If zero, it is the final week of the previous year.
            We determine that by asking for the week number of Dec 31.
        */
	if (weeknum == 0) {
		y = y - 1;
		iso_week_date_r(y, 364 + is_leap_year(y), iso);
		iso->day = wday;
	}
}
